The present disclosure relates to a sensing apparatus.
A sensor using an optical fiber is advantageous in that it is free of corrosion, exhibits high endurance, and has electromagnetic interference immunity. Particularly, an optical reflector such as an optical fiber Bragg grating is suitable for the use as a sensor because of its small size and high sensitivity to the temperature and strain.
Sensor networks using an optical reflector are classified into a wavelength division multiplexing (WDM) method in which a variation in center wavelength of each sensor is sensed with respect to strain applied from an outside and a variation in temperature, a time division multiplexing (TDM) method in which each sensor has the same center wavelength and a pulse string having a period that is longer than a distance between sensors is transmitted to the sensors to measure variation in output power reflected by each sensor, and a code division multiplexing (CDM) method in which an input light source is modulated using a pseudorandom number generator, the modulated light source is transmitted to each sensor, and then variation in autocorrelation value of a pseudorandom number returning to each sensor is measured.
The WDM method has been widely used in that it has a high resolution and may be easily realized compared with other systems, but has a limitation in that the number of sensors is limited because when a center wavelength of a sensor is changed by an external factor, the center wavelength of the sensor may not overlap a center wavelength of another neighboring sensor.
The TDM method has solved the limitation in the number of sensors by using sensors having the same center wavelength and an optical reflector having a low reflectivity to measure a variation in output power returning from each sensor. However, since the TDM method makes the design of a signal processor be complicated, it is expensive in price, and since the TDM method uses a single pulse, it has a limitation such as a low response rate, compared with other methods.
The CDM method may be realized by modulating a signal of an optical source by a pseudorandom code to transmit a code string to each sensor, and autocorelating the code string returning from each sensor. The CDM method may realize a demodulation circuit for a sensor system only with a low price electrical device without a high speed signal processing circuit.
However, since the related art CDM sensor uses a variable laser in order to increase the operation range, it requires a long scanning time and a delay for synchronizing the pseudorandom number code reflected and returning from each sensor and an originally generated pseudorandom code for autocorrelation. Also, since the variation in output power of autocorrelation value is measured, if the wavelength deviates from the center wavelength of the sensor, the measurement may be impossible.